DE2258661A1 - ACOUSTOOPTIC ARRANGEMENT FOR DEFLECTING AN OPTICAL BEAM - Google Patents

Description

Western Electric Company, Incorporated. Bonn 7-11-16

New York, N.Y., U.S.A. ' 9 R 8 R R

Acousto-optical arrangement for deflecting an optical beam

The invention relates to an acousto-optical arrangement for deflecting an optical beam, with a body made of paratellurite tellurium dioxide, a wave generating device for generating acoustic or elastic shear waves, which propagate along the EllO3 axis in the body, and with a Frequency modulation device which modulates the shear waves in such a way that they create an optical beam around one deflect the frequency of the shear waves related angle.

It is known that an acoustic beam of wavelength / \ in its interaction with a light beam of wavelength λ as a diffraction grating with the distance or the Lattice constant iV acts, which the light beam under a -Angle f deflects by

is given when Bragg's law is fulfilled. In equation (1), θ and ip are the angles between the incident ones and deflected beams of light) and acoustic Grating and V and f the acoustic speed and frequency, respectively.

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In this form of the Bragg angle deflection, the deflection angle <P always has the same size as the angle of incidence Θ. In order to scan the deflected beam over an angular range £ ± qj> , it is necessary to use a relatively narrow transducer of length L in order to spread the acoustic energy in an angular range A θ = A / L. Accordingly, the output light beam bundle is scanned with modulation of the acoustic frequency by an amount Af (the bandwidth) over an angle Λ ^ f, which by

is given, where η is the refractive index. It can be shown that the number of rays from the scanned beam .resolvable points N is equal to

N «| a f« tkf (3)

where D is the opening of the incident light beam and T is the access time, i.e. that of the light beam

to cross the acoustic wave / time is required.

These known acousto-optic light deflectors there are some problems attached. In particular for a given acoustic center frequency, the interaction

to enlarge, length L be reduced to the deflection angle ^, which in turn

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higher acoustic performance is required. In fact, that will acoustic performance at reasonable deflection angles and efficiencies in known permanent line devices so high that thermal stresses in the acousto-optic material strongly distort diffracted light beams.

In 1967, however, RW Dixon postulated that deviations from normal Bragg's diffraction occur when the incident light beam is given a direction of propagation perpendicular to the optical axis of a uniaxial birefringent crystal. In an article entitled "Acoustic Diffraction of Light in Anisotropy Media" in the IEEE Journal of Quantum Electronics , OE-3 , 85 (1967), he stated that one of these deviations is that the angle of incidence θ is no longer equal to the angle of deflection φ needs to be. He also pointed out that the deflection angle, plotted against the acoustic frequency characteristics of deflection devices using such birefringent media, has an extreme point (inflection point) at a frequency f 'at which dO / df = 0. By frequency modulating the acoustic wave around a center frequency that is equal to f., Dixon proved that larger diffraction angles φ could be achieved than before.

Unfortunately, the extreme point frequency f '(inflection point frequency) for most of the useful birefringent ones

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acousto-optic media in the gigahertz range in which acousto-optic Losses are very high. However, since the acousto-optical losses decrease with the square of the acoustic frequency, it would be appropriate to use an acousto-optic material with a lower f '. Also, the sound wave should in such a material have a relatively low speed, corresponding to the number of resolvable points To enlarge equation (3).

An acousto-optic material that is birefringent and has a relatively low velocity for shear waves propagating along the £ 110J direction is TeO ₅ , which was reported by N. Uchida et al in Journal pf Applied Physics , 40 , 4692 (1969). Using the teachings of Dixon, one calculates an extreme point frequency f ^β 1.17 GHz for an optical wavelength of about 0.44 m and a shear wave propagating _{in the Cliol direction in TeO 3.} Even at 1.15 yum, a value that is still within the optical passband (0.33 / um to 4.5 yum) of TeOp, f is about 0.45 GHz, i.e. still relatively high. Therefore, a deflecting device which would take advantage of the birefringence Te0 _? is suitably designed to experience undesirably high acoustic losses at almost all practicable optical wavelengths within their passband.

The problem of achieving high deflections of an optical beam at low acoustic powers becomes

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according to the invention in an acousto-optical arrangement thereby solved that an optical device is provided which polarizes the optical beam and the polarized Beams are an elliptical shape, the elliptical beam is directed so that it is in the Body spreads at an angle to its optical axis which is greater than zero that the angle of incidence of the optical The beam of rays over the acoustic frequency characteristic of the body has an extreme point which is different for different optical wavelengths occurs at different frequencies, and that the center frequency of the frequency modulation device approximately equal to the extreme point frequency corresponding to the wavelength of the optical beam is.

Show in the drawing

Fig. 1 is a schematic representation of an embodiment sbei game of the invention;

FIG. 2 shows an enlarged view of the TeO body from FIG. 1; FIG.

3 is an enlarged view of a LiNb (K converter for use in the embodiment according to FIG. 2;

Fig. 4 different directions of the light and acoustic beam, based on the optical axis;

Fig. 5 is a graphical representation of the incidence

and / or deflection angle versus acoustic frequency for both normal Bragg's Deflection (curve Ka)) and deflection according to the invention (curves IKa) and III Ca)); and

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FIG. 6 shows an illustration analogous to FIG. 5 with curves for three different optical wavelengths,

It is stated here for the first time that the inflection point frequency of paratellurite TeO »can be reduced to the useful ten-megahertz range, where acoustic losses are relatively low by using the optical activity of the material instead of its birefringence. The optical activity, which was explained in more detail in "Fundamentals of Optics", Ch. 28, McGraw Hill (3rd edition 1957) by Jenkins and White, is the property of a material through which elliptically or circularly polarized propagating parallel to the optical axis Light has one of two propagation speeds, which depends on the type of polarization. In one embodiment of the invention, the acoustic shear waves are directed so that they propagate along the C 110 ^ axis in TeO _, and an elliptically polarized light beam is directed so that it is at an acute angle y ¹ to the optical axis of TeO ₀ Crystal spreads. At the extreme point frequency, the deflected light beam runs essentially perpendicular to the ^ 10 * 3 axis. This combination leads to extreme point frequencies in the range from about 18 MHz to 82 MHz for optical wavelengths in the range from 1.15 to 0.4416 μπι. For a given angle of incidence θ and a given one

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Interaction length L, the diffracted light bundle can be deflected over a larger angular range than was previously possible (e.g. 2 degrees compared to 0.4 degrees in PbMoO ₄ ), or with a given bandwidth & f , the interaction length L can be made larger, whereby the required acoustic power is reduced and the problems associated with thermal stresses are eliminated.

It has also been found that, in order to achieve the aforementioned very desirable properties, paratellurite-TeO _{5 is} preferably used in the Czochralski single crystal pulling process

^ ₁ . the end

is grown from a starting material which is at least 99.9999 percent pure polycrystalline tellurium dioxide consists. In the following description under TeO " Understand Paratellurite TeOp.

In Fig. 1, a light beam deflection arrangement according to an embodiment of the invention is shown schematically. A linearly polarized light beam, which is generated by an L'aser 10, is passed through a quarter-wave plate 12 sent where it is converted into circularly polarized light will. If necessary, the plate 12 can be produced by a known compensator, for example a Soelil-Babinet compensator slightly elliptically polarized light can be replaced in order to avoid the direction running outside the optical axis.

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To compensate for the direction of the light beam, as will be described below. Next, the polarized light beam is passed through a cylindrical lens 14 to produce an asymmetrical (elliptical) intensity distribution which enlarges the aperture D and therefore increases the number of resolvable points according to equation (3). At the same time, an elliptically shaped beam reduces the beam cross-section in the direction normal to the deflection direction, which means that the transducer size can also be reduced in this direction. This gives the possibility of a TeO _? Use crystal of smaller size and lower acoustic energy.

The elliptically shaped and elliptically polarized light beam is focused on the TeOp body 18 by a lens 16. After the light beam and the Acoustic waves in the body 18 have interacted, the deflected light beam from a Telescopic lens system 28 focused on a consumer 30. The latter generally designates the diverse applications of acousto-optical deflectors or deflectors in, for example, real-time display systems, hard copy systems, and processing operations.

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As is shown precisely in FIG. 2, the TeO ₂ body 18 has at one end 18a a transducer 20 on which an electrode 22 is formed. The latter is connected to an oscillator 24 (FIG. 1) which can generate a frequency-modulated electrical output signal which, via a transducer 20, throws an acoustic or elastic shear wave in the direction of the vector K, ie the acoustic shear wave propagates in the ClIO ^ Direction and has a displacement svektor in the [ITlOUl direction. The LH0 ~ 3 direction is intended for shear waves because the acoustic velocity of TeO _{2 is} lowest in this direction, namely about 0.617 χ 10 cm / sec, whereby the number of points N that can be resolved by the deflector is according to equation ( 3) is increased. As shown in Fig. 4, the incident light beam propagates generally at a small angle -f * to the optical axis, where > ζ is non-zero and typically less than about 20 degrees. The plane P containing both the light and acoustic beams generally forms an angle with the optical axis, where β is typically less than about 20 degrees and can become zero. In addition, the incident light beam is at a small angle, θ (for example 1 to 3 degrees), the Bragg's angle, to the normal to the direction of the acoustic waves.

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During operation, the shear waves interact with the incident light beam, which is deflected by an angular range related to the acoustic frequency deviation Δf. It is essential that the angle of incidence of the light rays over the acoustic frequency characteristic shows an extreme point or a turning point at which the deflected bundle of rays is essentially at right angles to the acoustic bundle of rays, ie at right angles to the direction. It should be noted that for a given extreme point frequency the angles) f, β and θ are uniformly determined. If the light beam lies in the (ITO) plane (ß = 0), the angle f is equal to the angle Θ. The plane P determined by the acoustic and light beams can be rotated around the acoustic beam (ie β Φ 0). Then the light beam is further away from the optical axis, but still hits the acoustic waves at the angle Θ. In practice it has been found that the light beam can lie up to 20 degrees outside the optical axis. However, if the angle y between the light beam and the optical axis increases, two effects occur: (1) the extreme point frequency f is increased, whereby the acoustic losses increase; this may be desirable in some cases, since the extreme point or turning point frequency may be too small in a particular application, and (2) the deflection efficiency, i.e. the deflected part of the incident

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lumbar ray beam, is significantly reduced. For example, based on measurements with light at 0.4416 / μm in the (T10) plane ψ = 0, £ = θ = 3 degrees). that f = 82 MHz and that a deflection efficiency of 90 percent is achieved in the frequency band averaged around f. In contrast, measurements for p = 12 degrees, "^ = 13 degrees and θ = 4 degrees approximated an f '= 110 MHz and a maximum deflection efficiency of 40 to 45%.

The end 18b of the TeOp body opposite the end 18a radiating the acoustic waves is equipped with an acoustic absorber 26 ", which reduces reflections of the acoustic wave at the end 18b and thereby reduces subsequent interference effects with the light beam. For this purpose, the mechanical impedance as close to the absorber as possible to that of the _TeO? be adapted to the body. However, where a mismatch is, the end 18b 1103 may direction are formed at a small angle to the [so that the reflected acoustic waves are deflected from the zone in which the If necessary, the absorber 26 can be mounted in a cooling body 32, which is particularly useful in the case when the reflector is operated on a continuous wave basis.

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In one embodiment of the invention, the TeOp body was grown in the single crystal pulling method according to Czochralski, using commercially available TeO "powder with a purity of 99.9999 percent as the starting material. It has been found that the purity of the starting material is directly related to the quality of the grown TeO «crystal. In other words, significantly lower optical scattering losses and lower acoustic absorption losses were achieved if the TeOp starting material had the above-mentioned degree of purity, based on six nines, compared to the starting materials previously used with a degree of purity of 99.98 percent. Thus, at 50 MHz, the acoustic attenuation with a known TeOp is about 0.23 db // Js, while with the TeOp according to the invention there is only an attenuation of about 0.04 db / to ^ aJso almost a factor of six lower. Similarly, the losses at 100 MHz with known TeO _{2 were} 0.40 db / μs, while with the TeOp used according to the invention, losses of only about 0.13 db / us, that is, a factor of three lower, occurred.

Using these high-quality TeOp crystals, an acousto-optical light deflection device was set up, which currently the best deflector using lead molybdate was far superior. As shown in Fig. 2, the TeO body has the dimensions w = 8 mrn, h = 9 ram and 1 = 22 mm. An x-section LiNbO_ converter one

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Thickness of about 48 µm and cut as shown in Figure 3 was used. That is, the displacement vector was in the [J10 ~} direction and at 41 degrees or 49 degrees to the z-axis or to the y-axis in accordance with the proposals according to US Pat. No. 3,591,813 X-cut transducer acoustic shear waves along the [J103 direction into the TeO ₂ body 18. The absorber 26 comprised a magnesium block attached with epoxy to the end 18b that was cut at 2 degrees from normal. The mechanical impedance of magnesium is around 5.3 compared to · 3.6 for TeO _? . This mismatch caused an acoustic reflection of about 4 percent, which was deflected due to the inclined cut of the end face 18b.

In operation this was done by one working at 0.4416 Aim HeCd laser generated light beams with the help of a Soelil-Babinet compensator, slightly elliptically polarized and in the (T10) plane (/ 5 = 0 degrees) at an angle ^ = θ from about 3 degrees to the optical axis (i.e. to the LOOi} direction) of the TeOp body 18 directed. By frequency modulation The electrical output of the oscillator 24 was the acoustic frequency over a range of about 40 MHz swept up to about 120 MHz and the angles of incidence and deflection were measured and recorded as shown in FIG.

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Curve II (a) gives the angle of the incident light beam and curve III (a) the corresponding angle of the deflected beam for each acoustic frequency. In contrast, the curve Ka) shows both the angle of incidence and the angle of deflection (which are the same) for normal Bragg ¹ see deflection. It should be noted that a circularly polarized bundle of rays was chosen as the incident light in such a way that curve II (a) with the ... extreme point resulted for the incident bundle of rays.

At an optical wavelength of 0.4416 / µm, appears the extreme point at approximately f = 82 MHz, which is the point corresponds to where the angle of the deflected beam is zero degrees and appears at an angle of incidence (3.2 degrees), which is twice that of normal Bragg deflection (1.6 degrees). For example With a 60 percent bandwidth (i.e. 82 + 25 MHz) l must be the scattering of the incident light beam

(Δθ = A / L) about 1.2 degrees for normal Bragg's deflection however, only be about 0.2 degrees in the embodiment of the invention. Furthermore, an interaction length L can be used which is larger by a factor of 6, which means that the acoustic performance by a factor of about six reduced and the power density reduced by a factor of about thirty-six can throw. The one with the Problems associated with thermal stress become significant

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2258561

reduced, especially when the deflector is on is operated on a continuous wave basis. Using this deflection system, there were 500 resolvable points with a 10 us access time at 50 mW acoustic power and achieved 70 percent deflection efficiency. In comparison, a B, glue molybdate deflection system would be about 2 watts acoustic performance to achieve the same values, or it would have a deflection efficiency of only have about 2 percent with the same acoustic performance.

The curves Ka) and II (a) of Fig. 5 are reproduced in Fig. 6, where the acoustic frequency is plotted logarithmically. Furthermore, in Fig. 6 curves 1Kb) and IKc) showing the angles of incidence of light rays at wavelengths of 0.6328 µm and 1.15 µm, respectively, two typical Output radiation from a He-Ne laser Curves Kb) and Kc) are corresponding representations for normal Bragg's deflection. It should be noted that the wavelength of light from 0.4416 Aim to 0.6328 µm and 1.15 / µm increases while the extreme site frequency decreases from about 82 MHz to 40 MHz and 18 MHz.

Various possible designs result from these different extreme point frequencies. We then-

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assuming that 3 db acoustic attenuation via the light beam opening (ie in the [11Ö] direction) is permissible, the light beam opening D can be approximately 1 cm at 0.4416 / µm and an acoustic center frequency of about 82 MHz = f, and a 60 percent bandwidth would result in N = 750 resolvable points according to equation (3). The access time X would be about 16 µs. On the other hand, the opening could be D = 0.62 cm for N = 500 points and * - = 10 jus. The latter system would be particularly useful for real time display or hard copy systems. In contrast, with an optical wavelength of 0.6328 μm and an acoustic center frequency of approximately 40 MHz = f ', a higher capacitance N is possible, since the acoustic attenuation decreases with the square of the acoustic frequency. If one again assumes a 3 db acoustic attenuation over the opening and a 60 percent bandwidth, the opening D can be about 3.5 cm

rf

which leads to N = 1600 points and ^L = 60 jus. Further improvements can be achieved by using optical wavelengths in the infrared range, for example the 1.15 µm line of the He-Ne laser, at which f is approximately equal to 18 MHz. For example, N = 5000 points can be achieved at ~ = 500 / µs, provided that a TeO _? -Crystal of sufficient size can be grown (e.g. a 30 cm long crystal). As an alternative, N =

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500 points at ^ = 50 carrion in a 3 cm long stick TeOp can be achieved. The latter system is for example when using the relatively powerful 1.06 µm line of an Nd-doped yttrium aluminum garnet laser, particularly useful for treatment or processing operations. -

Even if the deflection of light at special wavelengths has been described above, it is important to that the new deflection system is useful for all optical wavelengths in the optical passband of Te0_, i.e. in the range of approximately 0.33 µm to 4.5 µm. Additionally a pair of deflectors can be used to deflect in orthogonal directions, or it can one of the deflectors for rapid unidirectional scanning and another such as a galvanometer can be used for slower scanning in an orthogonal direction. The latter arrangement can are particularly useful for forming a grid in a real-time display system.

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Claims (11)

-ie- 2253661 Claims Acousto-optical arrangement for deflecting an optical Beam of rays with a body made of paratellurite tellurium dioxide. a wave generator device for the generation of acoustic or elastic shear waves, which extend along the CHQ] Axis spread in the body, and with a frequency modulation device, which modulates the shear waves in such a way, that they deflect an optical beam by an angle related to the frequency of the shear waves, characterized in that an optical device (12,14) is provided which polarized the optical beam and the polarized one Beams are elliptical in shape, the elliptical beam is directed so that it propagates in the body at an angle to its optical axis which is greater than zero that the angle of incidence of the optical beam over the acoustic frequency characteristic of the body has an extreme point, which for different optical wavelengths occurs at different frequencies, and that the center frequency of the frequency modulation device (24) approximately equal to the extreme point frequency corresponding to the wavelength of the optical beam is. 309823/0815 2. Acousto-optical arrangement according to claim 1, characterized in that the optical bundle of rays is directed is that it spreads in the body at an angle to its optical axis which is not greater than 20 degrees is. 3. Acousto-optical arrangement according to claim 2, characterized characterized in that an acoustic absorber is so 'designed and arranged that he the shear waves after their Interaction with the optical beam attenuates. 4. Acousto-optical arrangement according to claim 3, characterized in that the absorber has one on the tellurium dioxide body Has attached magnesium body. 5. Acousto-optical arrangement according to claim 4, characterized in that that end of the tellurium dioxide body on which the shear waves impinge after their interaction with the beam, at an angle to the propagation Direction of the shear waves is arranged and the magnesium body is attached to this end. 6. Acousto-optical arrangement according to claim 5, characterized in that the wave generator device has a Has x-cut lithium niobate converter, which is connected to the 309823/08 15 Absorber carrying end is attached to the opposite end of the tellurium dioxide body. 7. Acousto-optical arrangement according to claim 6, characterized in that the tellurium dioxide body is low has optical scattering loss and low acoustic absorption loss and is a single crystal obtained by the single crystal pulling process after Czochralski from polycrystalline tellurium dioxide is bred with a purity of 99.9999 percent. 8. acousto-optical arrangement according to claim 7, characterized characterized in that the x-cut LiNbO ^ converter with a End of the body is so connected that its x-axis is along the £ l 10l direction of the body, its z-axis at about 41 degrees to the CT10] direction and its y-axis at about 49 degrees to the ΓΤίοΊ direction that a heat sink with the absorber is thermally coupled and that the transducer directs the acoustic shear waves so that they are in Spread the body in the [110} direction and in the LllQj Direction are polarized. 9. acousto-optical arrangement according to claim 8, characterized characterized in that the optical beam is generated by a He-Cd laser at 0.4416 / µm and the extreme point frequency is around 82 MHz. 309823/08 15 10. Acousto-optical arrangement according to one of the claims 1 to 8, characterized in that the optical beam is generated by a He-Ne laser at 0.6328 / µm and the extreme point frequency is around 40 MHz. 11. Acousto-optical arrangement according to one of the claims 1 to 8, characterized in that the optical beam is generated by a He-Ne laser at 1.15 / µm and the extreme point frequency is around 18 MHz. 309 823/08 15